A conventional charging apparatus for use with a portable electric tool includes, e.g., a battery pack 1 and a charger 2 on which the battery pack 1 is removably mounted, as shown in FIG. 8. The battery pack 1 includes one or more battery cells 3, and a cell voltage detection unit 5 for detecting each of the cell voltages of the battery cells 3, and outputting a cell voltage detection signal if any one of detected cell voltages is equal to or greater than a cell voltage threshold value V1. Further, the charger 2 includes a power supply circuit 4 for charging respective battery cells 3 of the battery pack 1 and a charging control circuit 6 for controlling the power supply circuit 4.
Charging control using the charging control circuit 6, as shown in FIG. 9, is performed in such a way as to perform constant current charging at a predetermined constant current value I1 until the cell voltage detection signal is received from the cell voltage detection unit 5 and then switch to constant voltage charging upon receiving the cell voltage detection signal. The constant voltage charging continues until the charging current becomes equal to or lower than a predetermined charging completion current value I2(<I1). As for such a quick charging scheme of switching from the constant current charging to the constant voltage charging, see for example Patent Document 1.
However, such type of charging apparatus, particularly in case where a lithium-ion battery is used for the battery pack 1, is problematic in that, if the high current, high voltage quick charging control is performed as described above in a low temperature environment, in which the temperature is equal to or lower than 0° C., the capacity of the battery pack is decreased, so that the life span thereof is greatly reduced. In order to solve this problem, it may be preferable to monitor the temperature of the battery pack and change the charging control scheme at low temperatures. However, no scheme for implementing such control has been proposed yet.
[Patent Document 1] Japanese Patent Laid-Open Application No.: 2005-317283